In the state of the art close to the invention, two types of operational amplifier are known, namely the voltage-mode operational amplifier and the current-mode operational amplifier. Each of the two types of operational amplifier generally includes one or two gain sections. Another type of operational amplifier also exists, namely the combined voltage input and current negative feedback amplifier. Since that type of amplifier is not part of the state of the art close to the invention, it is not discussed below.
A current-mode operational amplifier has a wide passband and good accuracy for defining its gain, but unfortunately its gain cannot take a high value, because the gain of a current-mode gain section cannot be high.
In current mode, amplification proper is generally based on using a translinear mixer. Unfortunately, with a translinear mixer, if the two bias currents applied to the two input nodes are modulated by a common-mode current, the gain, which is a differential gain that is proportional to the ratio between the two bias currents, is also modulated. In other words, unwanted common mode gain might be produced.
A voltage-mode operational amplifier offers high gain, but suffers from numerous drawbacks. The value of the gain is difficult to control. The energy consumption can be low only at the expense of speed. In the same way, to obtain a wide passband, high current consumption is required. Finally, with filters, attenuation of fast signals makes it necessary to use high bias currents.
In order to obtain high gain while retaining most of the advantages offered by current mode, a known solution consists in chaining together a large number of current-mode gain sections, in general inside the same operational amplifier. In this way, the advantages of current mode, namely, in particular, good gain accuracy, are combined with high gain obtained by means of the chain structure.
However, that known solution suffers from a major drawback, namely the risk of phase instability. On passing through each of the gain sections in the amplification system, the signal is subjected to phase rotation.
The total phase rotation at the output of a current-mode amplifier is equal to the sum of the phase rotations imparted by the various sections. Since the chain structure of the amplifier comprises a large number of gain sections, the total output phase rotation is generally high. It is clear that, in most cases, the total phase rotation gives rise to instability in the feedback-mode operational amplifier.
In order to counter such instability, and to have a wide enough output phase margin, it has been proposed to create a dominant pole by working at low frequencies. But being constrained to work at low frequencies only, which results in a loss of passband in the amplified signal, makes that solution unattractive because it removes one of the main advantages of current mode, namely-having a wide passband.
Moreover, known state-of-the-art current-mode amplifiers suffer from other constraints, such as, in particular, low current offset and low noise, and high speed.
The current offset, which occurs in addition to the difference in current to be amplified, results from imperfect matching of transistors, and it can be attenuated only by choosing "large" transistors on which it is easier to obtain good geometrical accuracy. In the same way, the noise generated by the transistors, which is dependent on the W/L ratio of the component (e.g. MOS), is lower for large transistors than it is for small transistors. Unfortunately, large transistors suffer from the drawback of being slow (because their W/L ratio is low), i.e. they can only work at low frequencies, and only small transistors, which are of simple shape, can work at high frequencies.
Therefore, a compromise must be made between firstly speed and secondly current offset and noise.